https://en.wikipedia.org/wiki/Siege_of_Constantinople_(674–678)
https://en.wikipedia.org/wiki/Siege_of_Constantinople_(717–718)
If one of the Umayyad sieges of Constantinople succeeded in conquering the city and defeating Byzantine forces, how far into Europe would the Caliphate occupy by 25 years later?
See the map below for options
Zone 1 in one of the dull gray colors is essentially what constituted Byzantine territory in OTL's 717, so the conquered territory is mainly Anatolia, eastern Thrace, several Greek islands, several coastal enclaves in Greece, Sicily, Calabria and Apulia. At this time, the Byzantine empire was very "sea-hugging" at this point in time having lost the interior of Greece, Thrace, all Macedonia and the interior of the Balkans to the Bulgars, Avars and Slavs.
If the Arabs conquered Constantinople in one of the PoDs, would the Arabs be only occupying coastal fringes of Europe 25 years later (in 700 ce or 745 ce)? That would be Zone 1 on the map.
Or would the Caliphate push further, occupying all of modern day Greece (Zone 2, pink)
Or extend up to the Danube and Morava rivers (Zone 3, green)
Or the Drina river (Zone 4, orange)
Or all of Illyria up to the Save (Zone 5, blue)
Or into Central Europe up to the Dniester, Carpathians and Alps (Zone 6, yellow)
...
